How to make this join with a TSQL query?

Question

I have a table called USERS that has a foreign key to the table GROUPS (a user can pertain to one or none GROUPS ). The table USERS also contains a column ISDELETED (a char column with T or F ).

I need a query to retrieve all the GROUPS and all the USERS that are not deleted, if all the users in a GROUP are deleted or no users are defined I need the query to return NULL for that GROUP .

I tried with the following query:

SELECT     GROUPS.*, USERS.*
FROM       GROUPS INNER JOIN
                  USERS ON GROUPS.ID = USERS.GROUPID
WHERE USERS.ISDELETED = 'F'

But this query does not returns the groups that are empty. SQL and me are not the best friends in world, some help will be great, thanks.

Answer 1

If you want all the groups, regardless of a match in the users table, you should use a left outer join:

SELECT     GROUPS.*, USERS.*
FROM       GROUPS 
           LEFT OUTER JOIN
           USERS 
           ON GROUPS.ID = USERS.GROUPID AND USERS.ISDELETED = 'F'

Answer 2

You should just need to do a left outer join -

SELECT     GROUPS.*, USERS.*
FROM       GROUPS LEFT OUTER JOIN
                  USERS ON GROUPS.ID = USERS.GROUPID
WHERE USERS.ISDELETED = 'F'

Here's a reference I like to use to remind myself of the differences in sql joins.

Answer 3

您需要使用LEFT OUTER JOIN运算符而不是INNER JOIN 。